1. Field of the Invention
The present invention relates to an optical filter which can change a wavelength characteristic representing a relation between transmittance and wavelength.
2. Description of the Related Art
In recent years, an optical pumping type of optical amplifier including an erbium doped fiber amplifier (EDFA) for amplifying signal light having a wavelength of 1.55 .mu.m band as a leading optical amplifier has become a level of practical application. In such an optical pumping type of optical amplifier, signal light and pumping light are introduced into a light amplifying medium doped with a rare earth element, and the signal light is amplified by the power of the pumping light.
In this kind of optical amplifier, an optical filter is used for various purposes. For example, a band-pass optical filter having a narrow passband is used to cut off light (spontaneous emission light and pumping light) other than signal light amplified in a light amplifying medium. Further, in applying the optical amplifier to a wavelength division multiplexing (WDM) system, a wide-band gain is required. Therefore, to make flat a gain characteristic of the optical amplifier representing a relation between gain and wavelength, an optical filter having a wavelength characteristic reverse to the gain characteristic. Since the gain characteristic of the optical amplifier and the wavelength of signal light are not always constant, an optical filter having a variable wavelength characteristic is desired.
Conventionally, an optical filter having a mechanical movable portion is known as the optical filter having a variable wavelength characteristic. In this kind of optical filter, the wavelength characteristic is varied, for example, by mechanically changing the angle of incidence of an input beam upon an optical interference film or a diffraction grating.
However, such an optical filter having a mechanical movable portion has a defect such that a high-speed operation is difficult and reliability is lacking. Further, in an optical amplifier to be applied to an optical repeater for long-distance transmission, the polarized condition of an input beam is not defined. Therefore, to make an output from the optical repeater constant, it is essential to use a polarization-nondependent optical filter such that transmittance is not dependent on the polarized condition of the input beam.